Ultra-wideband (UWB) technology is a short-range radio technology which is ideal for use in wireless personal area networks (WPANs). UWB technology provides data transfer rates of up to 480 MHz at a distance of 2-3 meters in realistic multipath environments whilst consuming very little power. Systems embodying UWB technology may be produced in a very compact form.
The UWB industry promoter, the MultiBand OFDM Aliance (MBOA) is developing specifications for a UWB solution using multiband OFDM for the physical (PHY) layer and the media access controller (MAC) layer and the interface inbetween for a diverse set of applications, such as real-time wireless video transmission, cable replacement (e.g. USB, 1394), direct print from digital cameras, wireless digital projectors, wearable computing, and wireless Desktops.
One approach for implementing UWB technology is the multi-band Orthogonal Frequency Division Multiplexing (MB-OFDM) technique. In this technique, the UWB signals for communications applications are specified to have a minimum bandwidth of 500 MHz within the 3.1 GHz to 10.6 GHz spectrum.
There are a number of problems in UWB system development, including extremely high bandwidth requirements (up to 500 MHz) and very low transmission power. These lead to a very hostile propagation environment with serious multipath fading and noise likely in conventional systems. Therefore, complicated signal processing is generally required to achieve successful data transfer.
In OFDM systems, the available spectrum is conventionally divided into a number of sub-carriers. By making all sub-subcarriers narrow band, they experience flat fading, which makes the equalization very simple. However, in the presence of strong echoes, some carriers will suffer deep fades, due to the destructive combination of the various reflections, while others will be enhanced by constructive addition. Coding and interleaving applied to OFDM may be used to average the local fadings over the whole signal bandwidth and over the time interleaving depth.
In a coded OFDM (COFDM) system, the received signal is noisier when it fades than it is when it is enhanced. This inequality in signal power will decrease the performance of, for example, a Viterbi decoder due to the reliability of received data differing from subcarrier to subcarrier. Therefore, it is necessary to estimate dynamically the data reliability in each subcarrier position. To attain close-to-optimal decoding performance, any coded OFDM system must rely on the use of channel state information (CSI) in a soft decision.
Several methods of using the CSI in a Viterbi decoder are known and a number of such conventional techniques and systems are described in the following documents:
“Performance analysis of Viterbi Decoder using channel state information in COFDM system” by Weon-cheol Lee, Hyung-Mo park, Kyung-jin Kang and Kuen-bae kim, published as IEEE Transactions on Broadcasting, Vol.44, No.4, December, 1998, pp 488-496; “A soft decision decoding scheme for wireless COFDM with application to DVB-T” by Yong Wang, Jinhua Ge, Bo Al, Pei Liu, ShiYong Yang published as IEEE Transactions on Consumer Electronics, Vol.50, No.1, February, 2004, pp 84-88; “A demapping Method Using the Pilots in COFDM systems” by Min-Young-Park and Weon-Cheol Lee, published as IEEE Trans. On Consumer Electronics, Vol.44. No.3, August.1998, pp. 1150-1153 and “Use of Linear Transverse Equalisers and channel state information in combined OFDM-equalization” by Simon Armour, Andrew Nix, David Bull published as IEEE proceeding, 2000, pp 615-619.
A popular method of calculating and utilizing CSI is proposed in Weon-cheol Lee, Hyung-Mo park, Kyung-jin Kang and Kuen-bae kim, “Performance analysis of Viterbi Decoder using channel state information in COFDM system.” IEEE Transactions on Broadcasting, Vol.44, No.4, December, 1998, pp 488-496. There are a number of problems with the method disclosed therein, firstly, the CSI generation and the corresponding post processing are highly complex. Secondly, when a bit interleaver is adopted in such as system, the accuracy of the CSI cannot be maintained for each bit. The reason for this is that two bits of one modulation symbol (with the same CSI) are deinterleaved, whereas, the bits which contribute to the new branch metric by using the same CSI do not in fact have the same CSI. The number of such bits depends on the coding rate R. Thus, in this case, only the average CSI can be used according to the bit deinterleaver rule. However, using the average value of the CSI in the Viterbi decoder does not give the best results.
In the publications by Yong Wang, Jinhua Ge, Bo Al, Pei Liu, ShiYong Yang entitled “A soft decision decoding scheme for wireless COFDM with application to DVB-T” (published as IEEE Transactions on Consumer Electronics, Vol.50, No.1, February, 2004, pp 84-88) and by Min-Young Park and Weon-Cheol Lee entitled “A demapping Method Using the Pilots in COFDM systems” (published as IEEE Trans. On Consumer Electronics, Vol.44. No.3, August. 1998, pp. 1150-1153) a CSI utilization method is disclosed. In this scheme, the overall data reliability is obtained by multiplying the CSI with the decision value from a demapper and this multiplied value is applied to the Viterbi decoder. In this case, the implication of the bit interleaver is clarified and full use is made of the CSI. However, this method requires a separate CSI generator and a combined reliability generator, which is costly and equalization achieved by this method remains complex.
Thus, there is a need for a method in which full use is made of the CSI whilst achieving improved system performance and which has a much lower complexity than a conventional minimum mean square error (MMSE) equalizer and a conventional least square (LS) equalizer.